The present disclosure relates to a communications system of noncontact type in which a communications terminal (or a transponder) containing no radio generating source transmits data with a device (or a reader/writer) that is a communications mate in a wireless manner and to a communications apparatus that is configured to operate as a transponder or a reader/writer in the communications system. More particularly, the present disclosure relates to a communications system and a communications apparatus that are configured to execute data communication on the basis of reflected-wave transmitting by use of the transmission of a non-modulated carrier from a reflected-wave reader and the absorption and reflection of received radio waves based on a terminal operation at an antenna of a reflector.
In detail, the present disclosure relates to a communications system and a communications apparatus that are configured to execute data communication between a transponder and a reader/writer that are arranged at an optimum communications distance and, more particularly, to a communications system and a communications apparatus that are configured to execute communication at an optimum communications distance for every application between the transponder and the reader/writer and, at the same time, prevent the interception by nearby communications terminals so as to ensure the security of transmission data.
A noncontact communications system called RFID (Radio Frequency Identification) is known as a communications system that is configured to transmit data by radio without having an own radio generating source. The RFID is also referred to as an ID system and a data carrier system. Commonly used worldwide is an RFID system or simply RFID, a recognition system based on high frequency (radio). The RFID system is made up of a transponder called a tag and a reader/writer for accessing the transponder. The transponder passively operates on the radio supplied from the reader/writer as an energy source and the reader/writer reads information from the transponder and writes information thereto.
The noncontact communications methods based on the RFID include electrostatic coupling, electromagnetic induction, and radio communicating, for example. With the RFID system based on radio communicating, the transponder has a reflector configured to transmit data by a reflected wave obtained by modulating a non-modulated carrier and the reader/writer has a reflected-wave reader configured to read data from a modulation reflected-wave signal received from the reflector, thereby executing reflected-wave transmission also called backscatter. Receiving a non-modulated carrier from the reflected-wave reader, the reflector modulates the reflected wave on the basis of antenna load impedance switching for example, thereby superimposing data onto the carrier. Namely, because the reflector requires no carrier generating source, a data transmission operation can be driven at low power dissipation. The reflected-wave reader receives the modulated reflected wave thus obtained and demodulates and decodes the received wave to obtain transmission data.
Basically, the reflector has an antenna for reflecting radio that is an incident continuous wave, transmission data generating circuit, and an impedance variation circuit for varying a load impedance of the antenna in correspondence with transmission data (Disclosed in Japanese Patent Laid-open No. Hei 1-182782). The impedance variation circuit is an antenna switch for switching between open and ground of the terminal of the antenna, for example. This antenna switch may be made up of a CMOS (Complementary Metal Oxide Semiconductor) transistor by building the antenna switch into a circuit module. It is also practicable to configure the antenna switch by a GaAs (Gallium Arsenide) IC (Integrated Circuit), separate from the circuit module, thereby realizing high-speed switching at low power dissipation. In the latter, a data transmission rate based on reflected-wave modulation is enhanced, while the power dissipation is suppressed below several 10 μW. Therefore, considering a power dissipation of about sever hundred mW to several W in a wireless LAN (Local Area Network), the reflected-wave communication can be said to be remarkably higher in performance than the average power dissipation of a general LAN (Disclosed in Japanese Patent Laid-open No. 2005-64822).
Because the transponder carrying a reflector only executes an operation of reflecting received radio, the transponder is not regarded as a radio station, so that this transponder can be advantageously handled as outside the regulations imposed on radio communication. While conventional noncontact communications systems use frequencies of several MHz to several hundred MHz (13.56 MHz for example), the reflected-wave transmitting can use a high-frequency band of 2.4 GHz (microwave) called ISM (Industry Science and Medical Band), for example, thereby realizing high-speed data transmission.
In one typical exemplary use of a noncontact communications system, a host device incorporating a transponder, such as a reflector, is arranged on the reading surface of a reader/writer, such as a reflected-wave reader as shown in FIG. 8, thereby executing information reading and writing on the transponder.
The transponder can obtain an electrical power by rectifying a carrier transmitted from the reader/writer; but this power is not enough for the transmission of large amounts of data at high frequencies. Namely, with the transponder, an electrical power for generating a carrier at the time of transmission is not required, while an electrical power necessary for a transmitting operation for modulating a reflected wave and a receiving operation for demodulating and decoding a modulated signal from the reader/writer is supplied from the host device.
For example, if the transponder is built in portable information terminals, such as a digital camera and a mobile phone with camera and a mobile terminal device, such as a portable music player, which are minimized in power dissipation and the reader/writer is built in information equipment based on stationary household appliances, such as a television set, a display monitor, a printer, a personal computer (PC), and a VTR (Video Tape Recorder), a DVD (Digital Versatile Disc) player, then bringing the transponder and the reader/write to proximity of each other allows uploading of image data taken by a mobile phone with camera or a digital camera to the PC in a noncontact manner, thereby storing, displaying, or printing the uploaded image data.
A memory card having capabilities of transponder, such as a reflector, is proposed (disclosed in Japanese Patent Laid-open No. 2006-216011). Such a host device loaded with such a memory card as a digital camera or a mobile phone can access the loaded memory card via a wired interface. On the other hand, an external device, such as a PC or a printer, can read data from a loaded memory card via a reflected-wave transmission route independently of the host device loaded with the memory card. Namely, a noncontact communications operation can be controlled out of control of the host device loaded with a memory card, so that host device need not install any driver software necessary for controlling noncontact communication since the memory card connects to an external device.
Data transmission between transponder and reader/writer requires to establish connection therebetween. For one of communications establishing procedures, a service entry sequence is proposed in which the reader/writer transmits a beacon frame at certain intervals to tell a service area of own station. (For example, see Japanese Patent Application No. 2006-270365 which has been already devolved to the applicant of the present disclosure)
FIG. 9 shows a communications control sequence for executing reflected-wave transmission between transponder and reader/writer by use of the above-mentioned service entry sequence.
The reader/writer periodically transmits a beacon frame to tell the service area of own station. Also, in an entry period to be provided after the transmission of a beacon frame, the reader/writer continues to transmit a non-modulated carrier for operating the transponder.
On the other hand, the transponder receives a beacon frame to know the existence of the reader/writer and uses the non-modulated carrier received within the entry period to return an entry frame in response to the received beacon frame.
FIG. 10 shows a sequence in which a communications operation is started between reader/writer and transponder by use of the service entry sequence shown in FIG. 9.
The reader/writer executes the transmission of beacon frames at certain time intervals in an intermittent manner. The transponder cannot receive beacon frames outside a radio reach range. However, when the transponder achieves in the radio reach range and a beacon frame comes, the transponder executes the processing of receiving the beacon frame.
On the basis of the information written to the payload of the received beacon frame, the transponder obtains information about communications frequency channels to be used and information such as the ID unique to the reader/writer. In order to establish connection with the reader/writer, the transponder returns an entry frame composed of modulated reflected-wave signal in an entry period by use of a non-modulated carrier transmitted from the reader/writer described above.
On the basis of the written contents of the entry frame transmitted from the transponder, the reader/writer obtains the information such as the ID unique to the transponder and communications parameters that can be set. In order to make communication with the transponder, the reader/writer transmits a connection request frame with specified information such as communications parameters written to the payload. In response to this connection request, the transponder returns a connection response frame with the information such as a connection result written to the payload, upon which the connection between transponder and reader/writer is established. While the connection is established, information is read from the transponder and written thereto by repeating the transmission of command frames from the reader/writer and the return of response frames by the transponder.
The reader/writer need not always continue to transmit beacon frames at certain time intervals. For example, the reader/writer can establish connection through the same connection request/response sequence as described above after the transmission of a beacon frame (or an irregular transmission request signal) when to read information from the transponder (to be more specific, every time an application for reading/writing data with the transponder is started up). Namely, the reader/writer can suppress the transmission of unnecessary beacon frames and the transmission of the non-modulated carrier during an entry period.
In the communications sequence shown in FIG. 10, a handshake is executed between the reader/writer and the transponder by use of a beacon frame and entry frame and then the reader/writer transmits a connection request frame, in response to which the transponder returns a connection response frame. Conversely, it is also practicable for the transponder to transmit a connection request frame, in response to which the reader/writer returns a connection response frame.
Today, a noncontact communications system, such as described above, executes an information read/write operation in a noncontact manner between the transponder and the reader/writer, thereby enhancing the convenience of use to expand the application of this system to the conventional magnetic cards, such as commuter passes and certification cards, and distribution systems.
Unlike wired communication, wireless communication is much more vulnerable to the risk of the interception of information that is carried by wireless communications systems, so that the communication routes must be encrypted. For example, if the non-contact communications system is used for the reading/writing of value information, such as the settlement in electronic money, or room access control, the ability of communication with terminals located in the proximity of the transponder and the reader/writer presents a serious problem in security.
For example, with wireless LAN systems, such as the IEEE 802.11 system, a scheme is introduced in which encryption keys, such as WEP (Wired Equivalent Privacy) and PSK (Pre-Shared Key) are held in common in the BSS (Basic Service Set) managed by a control station, thereby enabling the terminal stations accommodated in a network to have secure communications paths based on encryption. However, the installation of such a scheme on noncontact communication systems that are characterized by low-cost configuration is not realistic.
With noncontact communications systems based on electromagnetic induction, the induction field decreases in inverse proportion to the cube of distance, so that the risk of interception by nearby communications terminals is comparatively low. However, with applications in which the transponder is installed on a device providing an information source, such as a digital camera, a mobile phone with camera, or a portable music player, to transmit information content to the reader/writer of a display apparatus or an audio output apparatus for outputting image or sound, it is inconvenient to have the communications distance limited to proximity.
The noncontact communication based on reflected-wave transmitting uses radio waves that attenuate in reverse proportion to distance, so that the communications distance is comparatively long for enhanced user-friendliness, but at the cost of sufficient security measures for the transmission of value information.